Providing a high-stability reference frequency is an important part of many communication and navigation systems operating in environments subject to substantial variations in temperature. Typically, the reference frequency is provided using a transistor-driven oscillator circuit having a crystal oscillator or an inductance-capacitance (L-C) network to establish a selected operating frequency. For example, feedback oscillator circuits, such as the Colpitts, Pierce or Hartley types, operate by returning a portion of the output signal to the input to sustain oscillation by positive feedback. Achieving a reference frequency that is highly stable in such a transistor-driven oscillator circuit typically requires use of a temperature-controlled quartz-crystal oscillator, such as those manufactured and sold by the instant assignee, Rockwell International Corporation.
Many applications require that the high-stability reference frequency be provided using circuitry reduced in both size and power consumption. Typically, small size implies integrating as much of the circuitry as possible into an integrated circuit, and low power suggests using low supply voltages. Unfortunately, implementation in an integrated circuit using low circuit supply voltages degrades the performance of most oscillators used for high-stability applications, and as requirements for low power consumption and small size become more stringent, the degradation becomes more severe.
More specifically, when a transistor-driven oscillator circuit is powered using a low supply voltage, the base-emitter junction voltage (Vbe) of the oscillator transistor represents a significant portion of the available circuit voltage. This Vbe changes over the military temperature operating range specified by many military standards by almost fifty percent. In low-voltage applications requiring a highly stable reference frequency, such a Vbe change is unacceptable because it significantly changes the operating point of the circuit and negates the desired stability.
When a transistor-driven oscillator circuit is implemented in an integrated circuit, space is a primary concern. While transistors can be implemented in a relatively small silicon area or nano-acre, the area required for a resistor is relatively large and the demand for silicon increases with the value of the resistor. Therefore, it is desirable to implement such circuit designs in integrated circuits using as few resistors as possible and with small resistance values. Previously implemented high-stability crystal oscillator circuits, however, typically require a lot of resistance, using many resistors and/or using resistors having large resistance values. Usage of "real-estate" within the integrated circuit for such a resistance requirement eliminates silicon needed for other circuitry and can require significant compromises with respect to the stability of the reference frequency.
Accordingly, there is a need for a high-stability oscillator circuit which can be implemented using few resistors, small resistances and low supply voltages.